Generally, wireless local area networking (WLAN) devices must support two channel access methods, namely, contention-based access and polling driven access. Contention based access allows any wireless terminal or access point to capture the channel and transmit a data frame after monitoring the channel in accordance with carrier sensing procedures. When using the polling driven access procedure, on the other hand, all frame exchanges are initiated by an access point, either through a polling signal or by simply transmitting data from the access point to one of a plurality of stations. Either contention based or polling driven access techniques may be used to transport voice traffic between an access point and a plurality of wireless terminals, and extensions to both techniques are available to provide service differentiation between voice and data traffic classes sharing a single WLAN channel. However, the frame exchange sequences defined by both procedures are inefficient when used to deliver voice traffic between an access point and a plurality of stations. Additionally, neither technique offers adequate support for power savings operations in a wireless terminal that is transmitting and receiving voice traffic.
ANSI/IEEE Standard 802.11, 1999 Edition (hereinafter “the 802.11 standard”), defines two different methods of accessing the channel. Section 9.3 of the 802.11 standard defines a polling-based contention-free access method. Section 9.2 of the 802.11 standard defines a carrier sensing contention-based access method. Both types of access methods are described herein.
The WLAN frame exchange sequence using the contention-free access method is illustrated in FIG. 1. Under this method, a communication device such as a mobile terminal (MT), wakes up prior to each beacon transmission and waits to be polled by the access point (AP). The duration between transmission of the beacon and receipt of a polling message at a particular MT can depend on the loading level of the network as well as the scheduling algorithm at the AP. Along with the polling message, the AP also transmits a downlink voice packet. Then, the MT responds with an acknowledgement (ACK) to the downlink voice packet and an uplink voice packet destined to the AP. Upon successful reception of the ACK and uplink voice packet, the AP acknowledges the successful reception by sending an acknowledgement message to the MT. Following a successful frame exchange sequence, the MT can stop monitoring the channel and turn off its radio transceiver. It should wake up again to receive the next beacon transmission. Because the frame exchange sequence depicted in FIG. 1 has to be initiated by the AP, the MT has to be alert at all times waiting for the initiation. This causes a power drain on the MT's battery, which on average, is proportional to the number of MTs on the polling list for a particular AP.
The WLAN frame exchange sequence using a contention-based access method is illustrated in FIG. 2. Under this method, the MT starts contending for the channel in accordance with carrier sensing procedures defined in Section 9.2.1 of the 802.11 standard and random backoff time procedures defined in Section 9.2.4 of the 802.11 standard upon receiving a voice packet from the higher layer protocol. After winning the contention process and acquiring the channel, the MT transmits an uplink voice packet to the AP. After a small window of time has expired, represented by short interframe space (SIFS), the AP acknowledges receipt of the uplink voice packet by transmitting an ACK to the MT. Independently, in the downlink direction, the AP also starts contending for the channel upon receiving a voice packet from its network interface. Similarly, the AP transmits a downlink voice packet to the MT upon winning the contention. The frame exchange sequence is terminated when the MT responds with an acknowledgement message. Following the completion of either an uplink or downlink voice frame exchange, the transmitting station (e.g., AP or MT) invokes the random backoff procedure defined in Section 9.2.4 of the 802.11 standard prior to transmitting another voice frame. Because the downlink traffic flow is independent from the uplink traffic flow, the timing of downlink transmission is dependent only on the AP and the fixed network. Because the MT has no control over this timing, it has to remain alert at all times waiting for the downlink transmission, thereby causing a drain on the MT's battery. In addition, it is inefficient to allow the AP and MT to operate independently, which can result in additional contention between downlink and uplink traffic transmissions.
Thus, there is a need for an improved frame exchange sequence to address the drawbacks of the prior art.